An Si (silicon)-based field effect transistor mainly used in a current semiconductor device is of a normally-off type. A normally-off field effect transistor is such a transistor as being rendered conductive when a positive voltage is applied across a gate and a source and being rendered non-conductive when a positive voltage is not applied across the gate and the source.
Meanwhile, a GaN (gallium nitride)-based field effect transistor for which researches for practical use have been advanced in order to achieve such characteristics as a high withstand voltage, low loss, high-speed switching, and an operation at a high temperature, is of a normally-on type. A normally-on field effect transistor has a negative threshold voltage, and it is rendered non-conductive when a gate-source voltage is lower than a threshold voltage and rendered conductive when a gate-source voltage is higher than a threshold voltage.
When such a normally-on field effect transistor is used in a semiconductor device, various problems such as use of a conventional gate drive circuit being not permitted arise. Then, it has been proposed to connect a normally-on first field effect transistor and a normally-off second field effect transistor to each other in series to configure a normally-off composite semiconductor device.
Further, in order to prevent the normally-off second field effect transistor from breaking down due to a drain-source voltage of the second field effect transistor becoming high, there is also a method of connecting a Zener diode between the drain and the source of the second field effect transistor so as to restrict a drain-source voltage to a voltage not higher than a withstand voltage of the second field effect transistor (see, for example, Japanese Patent Laying-Open No. 2006-324839 (PTL 1)).